Twitter faces a fundamental challenge: how to keep growing advertising revenue at a fast clip when adding new users is getting more challenging.

The social network outpaced Wall Street’s expectations in the second quarter, posting total revenue of $502.4 million for the period, compared to analyst estimates of around $481 million.

But take a few steps back and you see that Twitter’s pace of growth has steadily come down from its stratospheric heights over the past year.

Ad revenue growth slowed from 129% in the year-ago quarter to 72% in the first quarter of 2015 and was 63% in the second quarter.

Now, obviously the law of large numbers dictates that growth is going to slow naturally over time in any company as the base number (its revenue) gets larger.

But Twitter’s struggle to attract new users is going to also place increasing pressure on ad revenue growth. It’s simple, really: the company can only squeeze a finite amount of ad dollars from its users each quarter before it risks alienating them by inundating them with ads.

During Twitter's earnings call on Tuesday the company's CFO, Anthony Noto, said its ad growth during the second quarter was driven by a number of factors, including a growing advertiser base, an increase in ad engagement, an increase in the number of ads it's inserting across its service, and the introduction of new ad products such as auto-play video advertising.

Interim Chief Executive Jack Dorsey said the company showed "good progress in monetization" during the quarter, but added it's not satisfied with its growth in audience.

Not including users that signed up for the company's "SMS fast followers" product, which allows users to get updates by SMS without signing up for an account, Twitter attracted only 304 million monthly active users during Q2, compares with 302 million during Q1.

To make matters worse, Mr. Noto said the company doesn't expect to see sustained, meaningful growth in monthly users any time soon. "In the near term organic growth will be slow", Mr. Noto said.

Twitter's ad revenue growth is still stronger than that of its major rival Facebook, however, which saw its ad revenue increase 46% during the first quarter.

Of course, Facebook's ad business and audience is significantly larger than Twitter's. The company attracted $3.3 billion in in revenue from advertising alone during the first quarter, and counted 1.44 billion active users as of March 31 2015, for an increase of 13% year-over-year.